The present invention generally relates to integrated circuits, and, more particularly, to a charge pump.
Charge pumps are used in integrated circuits (ICs) to provide operating supply voltages. Charge pumps also are used as current sources and current sinks to charge and discharge, respectively, internal nodes of ICs. In a typical application, a charge pump is used in a phase-locked loop (PLL) that includes a phase detector, a loop filter, and a voltage-controlled oscillator (VCO). The charge pump charges and discharges a capacitor of the loop filter to achieve a desired voltage on the capacitor based on up and down signals generated by the phase detector. The up and down signals indicate the phase difference between a reference clock signal and an output signal of the PLL generated by the VCO.
However, current mismatch between the charging and discharging currents leads to an offset between the reference clock signal and the PLL output signal, which impacts the performance of the IC. Further, the PLL bandwidth and the phase margin depend on the values of the charge pump current. Thus, to provide a large variation in the charge pump current, the charge pump needs to be programmable.
A known technique to reduce the mismatch between the charging and discharging currents is to include a replica bias circuit and a differential amplifier in the charge pump. The replica bias circuit and the differential amplifier ensure that the drain-to-source voltages of source transistors of the replica bias circuit are substantially equal to that of source transistors of the charge pump. The replica bias circuit and the differential amplifier also ensure that the drain-to-source voltages of sink transistors of the replica bias circuit are substantially equal to that of sink transistors of the charge pump. This reduces the mismatch between the charging and discharging currents. However, the charge pump still cannot provide a large variation in the charging and discharging currents, i.e., the charge pump does not enable the enhanced programmability of the charging and discharging currents.
A known technique to overcome the aforementioned problem is to include multiple source and sink transistors in the charge pump for charging and discharging the loop filter capacitor. The source and sink transistors can be switched on and off using external signals. However, in this technique, the current through the replica bias circuit does not change as desired, resulting in a voltage drop mismatch between the source transistors of the replica bias circuit and the source transistors of the charge pump core circuit, as well as a voltage drop mismatch between the sink transistors of the replica bias circuit and the sink transistors of the charge pump core circuit, which causes a mismatch between the two currents to resurface.
Another known technique to overcome the aforementioned problem is to use programmable transmission gates in the charge pump core circuit that connect the charge pump core circuit source and sink transistors, The transmission gates then are driven by the up and down signals and inverted up and down signals provided by the phase detector. This results in capacitive loading of the up and down and inverted up and down signals, which in turn results in additional channel charge injection and clock feed-through problems in the charge pump. This technique also causes a mismatch in charge pump input signals, i.e., the up and down signals and the inverted up and down signals.
It would be advantageous to have a programmable charge pump that can provide a large variation in the charging and discharging currents without causing a significant mismatch between the two.